Traditional vehicle suspension systems have included springs for supporting a sprung mass (i.e. the chassis) above an unsprung mass (i.e. the wheels and axles) and shock absorbers for damping relative vibration of the sprung and unsprung masses.
A traditional shock absorber utilizes a piston moving through a cylinder containing hydraulic fluid. The piston has a plurality of orifices to permit fluid to flow through the piston as it moves. The damping force generated by the shock absorber depends, inter alia, on the piston velocity and orifice size.
To improve vehicle handling and ride comfort, a variety of systems have been proposed wherein the shock absorber orifice size and/or geometry is controllable to adapt to varying road conditions. Additionally, so-called fully active suspensions have been proposed wherein a single hydraulic cylinder replaces the shock absorber and spring at each wheel. Additionally, a system has been proposed wherein the pressures in the compression and rebound chambers of the dampers are controlled by means of pressure regulators. The regulators are controlled either via a computer processor or a hydraulic/mechanical arrangement. For improved ride, in response to the control input, the damper provides a constant compression and/or rebound resistive force, which force is determined by the control system for each damper based upon the particular ride condition (acceleration, deceleration, cornering, bumps and the like). With such systems, for example, nosedive may be prevented upon braking by controlling the front dampers to provide a constant force resisting compression and the rear dampers to provide a constant force resisting rebound or extension. With any typical vehicle, braking results in a "weight transfer" from the rear wheels to the front wheels. This produces a relative compression of the front springs and extension of the rear springs, resulting in nosedive. If the weight transfer is, for example, an increase of 500 pounds to the front end of the vehicle and a decrease of 500 pounds to the rear end of the vehicle, a vehicle equipped with a constant-force system can set the pressure in the compression chambers of the forward dampers to yield the necessary 500 pounds (250 per damper) of resistive force so that the forward springs do not significantly compress. Conversely, the rebound chambers of the rear dampers provide a force resisting rebound of 250 pounds per damper so that the vehicle maintains a horizontal orientation.
If, under these conditions, the constant-force suspension-equipped vehicle hits a sharp bump or the like, so as to otherwise increase the pressure in the compression chamber, the pressure regulation provided by the system allows for a blow-off of fluid from the compression chamber so that the constant force is maintained.
Such systems are described in greater detail in James M. Hamilton, "Computer-Optimized Adaptive Suspension Technology (COAST)," IEEE Transactions on Industrial Electronics, Vol. IE-32, No. 4, November 1985, and U.S. Pat. Nos. 4,634,142 and 4,722,548. The disclosures of these three documents are incorporated herein by reference.
There are, however, a few problems that must be dealt with in the constant-force design. One problem is the lack of force when the direction of travel of the damper is wrong (i.e., not being able to provide a force in the same direction as the wheel/damper travel). If the vehicle hits a bump when braking forces are applied, after the upward portion of the bump is reached, the wheel will start to travel in the rebound direction so that the compression forces cannot be maintained and can return only after the wheel reaches its lowest position and returns to the compression direction once again. Another problem is finding an electrically controllable pressure-regulating valve that has sufficient size (orifice flow area for very high flow rates experienced when hitting bumps at high speed) at a reasonable size and cost. There are large flow valves that are controllable, but they have many problems for the present applications, including large size, high hysteresis in their pressure regulation, expensive and complex feedback systems, and relatively slow response times.
It is an object of this invention to efficiently solve the problem encountered when the control forces are momentarily lost during damper movements in the undesired direction.
It is a further object of this invention to solve the valve performance issues while at the same time reducing their cost and size.